1. Field of the Invention
The present invention relates generally to an Ultra Wide Band (UWB) wireless transmission technique, and more particularly to a UWB transmitting and receiving device for removing an unnecessary carrier component in its transmission signal spectrum, which is an expected requirement for implementing the UWB transmitting and receiving device.
2. Description of the Related Art
Standards currently proposed in the UWB standardization use a band pass type signal spectrum, which is different from the conventional impulse radio UWB transmitting/receiving device, in order to meet spectrum emission mask requirements proposed by the US Federal Communications Commission (FCC). Accordingly, the configuration of the UWB transmitting and receiving device is expected to differ slightly from that of the conventional heterodyne or homodyne type. By considering a bandwidth of at least 500 MHz as a characteristic of UWB signals, it is expected that the homodyne type, rather than the heterodyne type, will be dominant as a practically applicable implementation type.
When the conventional homodyne transmitting and receiving technique is applied directly to the UWB system, a certain amount of carrier frequency components, which are unrelated to signals originally intended to be transmitted, are leaked into the signal spectrum of the transmitter of the UWB transmitting and receiving device. The carrier frequency leakage is caused by limited physical characteristics of a modulator in the UWB transmitting and receiving device, and it is practically impossible to reduce the carrier frequency leakage to zero.
Generally, isolation between RF (radio frequency) and LO (local frequency) ports of a modulator used in the homodyne transmitter is in the range of 20 to 40 dBc with respect to driving power of the local frequency port. More specifically, 40 dBc can be considered the upper limit achievable by the current technology.
It is prescribed by the U.S. FCC that carrier frequency leakage of the UWB transmitting and receiving device be maintained at −41.5 dBm/MHz or less in bands from 3.1 to 10.6 GHz. Actual driving power of the local frequency port of the modulator achievable by the current technology is in the range of −5 to 0 dBm. When it is assumed that isolation between the LO and RF ports is at the best condition (i.e., 40 dBc), the carrier frequency leakage in the transmitter is in the range of −45 to −40 dBm, which violates the FCC regulations.
It is known in the art that the carrier frequency leakage must be maintained at −20 dBc or less with respect to the signal power to maintain a Signal-to-Noise Ratio (SNR), which can be demodulated in the receiver. Taking this into account, it is required that the isolation in the transmitter be at least 60 dBc, but it is practically impossible to achieve this value solely by the RF circuit design technology.